1. Field of the Invention
The present invention relates to a sample-and-hold circuit for generating a variable sample signal of a power converter and a method thereof, and particularly to a sample-and-hold circuit and a method thereof that can generate a sample signal changed with a load coupled to a power converter.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a power converter 100 according to the prior art. A control circuit 102 of a primary side PRI of the power converter 100 is used for controlling an output voltage VOUT of the power converter 100 by detecting a voltage VFB corresponding to an auxiliary winding 104. Because a voltage of the auxiliary winding 104 is positive and negative in turn, the voltage VFB corresponding to the auxiliary winding 104 needs to be sampled at a correct time. Generally speaking, the sample-and-hold circuit 108 generates a sample signal SS to a comparator 107 of the control circuit 102 after a power switch 106 of the primary side PRI of the power converter 100 is turned off for a period of time, wherein VREF is a reference voltage. However, when a load 110 coupled to a secondary side SEC of the power converter 100 is different, a discharge time of the power converter 100 is also different, so that the sample-and-hold circuit 108 utilizes a fixed sample time to sample the voltage VFB makes a regulation effect of the control circuit 102 be poor, and the control circuit 102 further incorrectly samples the voltage VFB when the load coupled to the secondary side SEC of the power converter 100 is light.